The folates are ubiquitous to nearly all forms of life. Humans and many other animals lack the capacity to make their own folate which thus is an essential vitamin, one type of essential nurient. Anemia especially during pregnancy and in the geriatric population was an early indication of a dietary requirement for folate. A major function of folate is to remove one-carbon units from molecules being metabolized and then deliver them to molecules being synthesized. As an example. folate participates in the formation of the nucleic acids. Further, the activity of DNA is controlled, in part, by methylation, and the primary methylating agent of the body (S-adenosylmethionine) is made in a metabolic cycle involving a folate. Many studies have, therefore, focused on the relationship of folate status to cancer susceptibility, especially colorectal adenoma.
The importance of folate to proper growth is clearly evident in the occurrence of neural tube defects in newborn infants. Reports from several countries have shown that a majority of such cases are associated with low folate levels in the mother. The incidence of these defects as well as of cleft lip/palate is considerably reduced when women are given folic acid (I) starting early in pregnancy. Recently, a significant correlation has been discovered between vitamin deficiency, especially of folate, and peripheral vascular disease, a major cause of death. A high percentage of individuals with this affliction have abnormal blood levels of homocysteine, a precursor to methionine in the folate dependent step of the S-adenosylmethonine cycle. Folate deficiency has also been linked to defective maturation of a number of different cell types, to nervous system disorders, and to decreased immune response.
The clear relation of folate intake to health has caused many governmental agencies around the world (such as the U.S. National Reasearch Council) to specify a recommended dietary allowance ("RDA") for folate. In the U.S. these values are used by the Food and Drug Administration to establish the Reference Daily Intake ("RDI") that is listed on food labels, currently 0.4 mg for adults. The highest daily amount of folic acid recommended by a country is 2.0 mg for healthy adults. Many products are available that contain RDI or near RDI levels of folic acid (I) including most daily multiple vitamins. These can be purchased in solid (eg. tablet, capsule, or powder) or in liquid formulations, both over-the-counter and by prescription. In the U.S. folic acid (I) is also available by itself typically at a dosage of 0.4 mg, but also up to 0.8 mg in health food stores. Many complete diets, infant diets, dietary supplements and weight loss products also contain folic acid (I). In some countries folic acid is added to specific food types as determined by health officials to provide adequate folate to the general population without risking excess consumption. Many breakfast foods, such as cereals, cereal bars, breakfast drink mixes, breakfast bars and toaster pastries have folic acid (I) added at a modest fraction of the RDI, typically 10-50% of the adult value per serving. In many of these uses the folic acid (I) is accompanied by other vitamins, sometimes at RDI dosages, but also at lower or much higher levels. Frequently, though not always, essential mineral nutrients are also present. Further, many products also include compounds hypothesized to have health related value, but which either have not been officially recognized as effective, or for which optimal amounts have not been set. Products such as those described above are meant to fill an important and wide spread need for folate, especially among those whose dietary habits would otherwise preclude intake of a sufficient amount of this vitamin.
Folic acid (I) is a component of many animal and pet foods. It is also included in powders or liquids used as animal feed supplements, often in combination with other nutrients. For example, the National Research Council (NRC) recommends diets containing 0.2 mg and 1.0 mg of folic acid (I) per kg of dry diet (assuming 5 kcal metabolizable energy per gram) for dogs and cats, respectively. For chicks the NRC has recommended 0.55 mg folic acid per kg of diet, although recent literature suggests that the optimal value is about three times higher than this.
The form of folate currently added to all commercial vitamin preparations or which is added to foods, folic acid (I) (also known as pteroyl-L-glutamic acid), is not one of the major forms found in natural fresh foods. The structure of folic acid (I) differs from the most abundant natural folate in several aspects. First, the side-chain of natural folates in almost all fresh foods contains more than one L-glutamic acid moiety. Frequently, five to seven (but covering a considerable span of more or fewer) of this amino acid are linked together into a polyglutamate chain. It is well known, however, that the primary form by which folates are absorbed has only a single glutamate residue. Cleavage of the extra glutamates of dietary folates is usually accomplished by an enzyme in the digestive tract. In this aspect folic acid (I) is not at a disadvantage in comparison to naturally occurring folates.
The second difference between folic acid (I) and natural folates is that whereas the pteridine ring of the former (I) is fully oxidized, natural folates in fresh uncooked foods are mostly present as the tetrahydro forms. Almost all of the known physio-logical functions of folate are performed by tetrahydrofolic acid, (6S)-FH.sub.4 (II), or by a one carbon derivative of it illustrated as follows: 5-methyl-(6S)-FH.sub.4 (III), 5-formyl-(6S)-FH.sub.4 (IV), 10-formyl-(6R)-FH.sub.4 (V), 5,10-methylene-(6R)-FH.sub.4 (VI), 5,10-methenyl-(6R)-FH.sub.4 (VII), and 5-formimino-(6S)-FH.sub.4 (VIII). The structural formula for each of these compounds is provided below. ##STR1## There is no known direct cofactor function for folic acid (I) itself in humans. Some (6S)-tetrahydrofolic acid polyglutamate is found in plants or animals, but the majority of folate is polyglutamate forms of either 5-methyl-, 5-formyl-(6S)-tetrahydrofolic acid, and in some cases 10-formyl-tetrahydrofolic acid. Presumably, most of the folic acid (I) found in biological food sources results from oxidation, especially on storage. When folic acid (I) is absorbed by the digestive tract it is eventually reduced to active (6S)-tetrahydrofolic acid (II) by the enzyme dihydrofolate reductase.
The oral bioavailability of folic acid (I) has been shown to be widely variable. The literature contains reports of individuals having poor intestinal uptake of folic acid (I) who respond normally to intramuscular injection of folic acid (I), or had normal serum folate status prior to any folic acid challenge. Several small scale investigations in which the values have been averaged have concluded that the oral uptake of several of the reduced folates is similar to folic acid (I). However, there is reason to believe that a segment of the population possesses adequate oral response to reduced folates, but not to oral folic acid (I).
5-Formyl-tetrahydrofolic acid (also known as leucovorin or folinic acid) has long been used in therapeutic doses for several diseases. Examples include rescue from the toxicity of methotrexate chemotherapy, and the synergistic combination with fluorouracil for treatment of various cancers. It is also given to treat acute anemia not due to B.sub.12 deficiency. 5-Methyl-tetrahydrofolic acid in high doses (for example, 50 mg/day) has been patented for treatment of depression (and other neurological disorders) (EP382019 and EP388827 to Le Grazie 1990, and EP482493 to Le Greca 1992).
That reduced folates have been overlooked as an improved source for providing the RDA level is in part due to the stereochemistry of these compounds. In addition to the single chiral center of the L-glutamate chain in folic acid (I), the tetrahydrofolates contain a second stereochemical center at carbon-6. Chemical reduction of folic acid (I) produces a nearly racemic mixture of the two isomers at this position. This is in contrast to the reduced folates found in nature which all consist of a single diastereoisomer, all having the same L-configuration at carbon-6. (Compounds II--VIII are shown as the natural isomer). For many years only the racemic 6(R,S) mixture of 5-formyl-tetrahydrofolic acid (leucovorin) has been used for therapy of diseases. Recently, however, concern over the possible effects of the unnatural isomer component has resulted in the commercial introduction of the pure natural isomer for these high dose disease treatments by Lederle, although at very high cost. Most therapeutic regimes utilizing leucovorin last a few weeks or perhaps months. The effect of a long term exposure to the unnatural isomer of reduced folates is unknown. For example, although little 5-formyl-(6R)-tetrahydrofolic acid is absorbed, there is considerable uptake of the unnatural isomer of 5-methyl-tetrahydrofolic acid by the intestinal tract and other cells of the body which with continuous intake may lead to adverse consequences.
Until recently, processes for making the natural isomer of reduced folates have been limited in scale, or costly, or both. These include chromatographic separation, enzymatic reduction, and fractional crystallization. The use of reduced folates as a daily source of vitamin requires a method that is applicable to large scale production of the natural isomer having high purity at a cost that will not place a burden on the average consumer.